In the manufacture of semiconductors, semiconductor devices are produced on thin disk-like objects called wafers. Generally, each wafer contains a plurality of semiconductor devices. The importance of minimizing contaminants on the surface of these wafers during production has been recognized since the beginning of the industry. Moreover, as semiconductor devices become more miniaturized and complex due to end product needs, the cleanliness requirements have become more stringent. This occurs for two reasons.
First, as devices become miniaturized, a contaminating particle on a wafer will occupy a greater percentage of the device's surface area. This increases the likelihood that the device will fail. As such, in order to maintain acceptable output levels of properly functioning devices per wafer, increased cleanliness requirements must be implemented and achieved.
Second, as devices become more complex, the raw materials, time, equipment, and processing steps necessary to make these devices also become more complex and more expensive. As a result, the cost required to make each wafer increases. In order to maintain acceptable levels of profitability, it is imperative to manufacturers that the number of properly functioning devices per wafer be increased. One way to increase this output is to minimize the number of devices that fail due to contamination. Thus, increased cleanliness requirements are desired.
One method by which the industry increases the cleanliness of wafers during processing is by introducing megasonic energy to the surface of the wafers during the cleaning step. Applying megasonic energy can enhance particle removal from semiconductor devices during cleaning processes. However, it has been discovered that applied megasonic energy can also damage the semiconductor devices being cleaned. The composition of the cleaning solution, including the quantity and composition of any gas dissolved in the cleaning solution, used in a megasonic cleaning process can affect cleaning efficiency and the amount of damage caused to the wafer. The prior art teaches that cleaning solutions containing supersaturated levels of gas are undesirable for wafer cleaning processes.
For example, U.S. Pat. No. 5,800,626 (the “'626 patent”) teaches that the cleaning solution should be partially saturated, e.g., 60-98% with gas, in order to achieve the best cleaning results. The '626 patent teaches that a lower saturation limit of 60% is required in order to maintain good cleaning performance. The '626 patent further teaches that too much gas in solution can create defects in silicon surfaces. Therefore, the cleaning solution should not be more than 98% saturated.
U.S. Pat. No. 6,167,891 (the “'891 patent”) teaches that a 100% saturated solution provides optimal cleaning efficiency. According to the '891 patent, under-saturated and supersaturated solutions provide significantly decreased cleaning efficiencies. The '891 patent attributes poor cleaning efficiency at supersaturated conditions to the formation of excessive gas bubbles in the solution that absorb the megasonic energy before it reaches the wafer surface. The '891 patent further teaches that for heated cleaning solutions, the solution must be partially degassed at low temperature before being heated in order to avoid supersaturation at the elevated temperature.
U.S. Pat. No. 5,849,091 (the “'091 patent”) teaches that an air/liquid interface across the wafer surface is critical to enhancing cleaning. However, the inventors of the '091 patent teach that the best method of forming the air/liquid interface is by directly injecting gas into the cleaning solution across the wafer surface.
U.S. Pat. No. 6,039,814 (the “'814 patent”) teaches that minute bubbles within the cleaning solution disrupt the propagation of sound, resulting in reduced cleaning efficiency. The '814 patent also teaches that bubbles create flaws in the wafer surface. The source of the bubbles is gas dissolved in the cleaning solution. Therefore, the '814 patent teaches, dissolved gas concentration in the cleaning solution should be below at least 5 ppm, and preferably below 3 ppm.